It looks like in Scala 2.13.12 the get method of a Map becomes case sensitive, so I had to add this to fix this test that was failing in this line

I was looking into that because this seemed a bit strange to me, to have that code failing solely by upgrading to Scala 2.13.12. (Sorry for the long comment, I just wanted to know myself what was going on here)

TLDR: Turns out this is not a bug in Scala and also they did not really change expected behaviour (tough they did change some internals which causes your code to fail). The actual problem with this code was that you assumed that map.get here is case insensitive, which isn't defined somewhere and you can not expect that.

If you look at the code:

The reason your code worked before was pure coincidence. Calling toMap

request.headers.toMap

here again just gives you a "stupid" map. Turns out, in Play this gives you a TreeMap implemention with a CaseInsensitiveOrdered ordering, see here:

val builder = TreeMap.newBuilder[String, Seq[String]](CaseInsensitiveOrdered)

Now, calling mapV on this method "transforms" this map, you can see that here:

  def mapV[U](f: map.V => U): Map[map.K, U] = map(coll).toMap.transform { case (_,v) => f(v) }

The toMap call here (which basically is toMap on the given map) in that line is calling Scala's internal code which in Scala 2.13.11 would return just the same instance that was given already, meaning it just returns the exact same map with the internal CaseInsensitiveOrdered ordering.
In Scala 2.13.12 however they changed some internals (because of other reasons) you get back a new instance of Map which however does not have the same ordering anymore. So the case-insensitive ordering from Play's internal code was lost and that is why your code breaks. Actually this is totally ok, because toMap does not make any promises which map it returns, not does it make promises on ordering.

The internal code they changed in Scala can be seen here, it's the from method:
In Scala 2.13.11 the matching case was:

case m: Map[K, V] => m

which just returns the same instance again (which was the same TreeMap with the ordering)
In Scala 2.13.12 however that case was removed and the code falls back to:

case _ => newBuilder[K, V].addAll(it).result()

which creates a new map, but as you can see the newBuilder does not get an ordering passed (turns out it then returns an Map2).

Thanks for the comment! This makes sense

cc @rtyley I think you will be interested in this!

This looks fine!